Created attachment 42289[details]
fbo-srgb.softpipe.png
mesa: 8af583f6e844ef2d173eb3c5c2e378cd1da018a8 (master)
The piglit fbo-srgb test passes on softpipe but looks different than from other drivers. On softpipe, three squares are displayed at (0, 0), (20, 0) and (30, 30). On NVIDIA, ATI, Apple, and swrast only the square at (20, 0) is drawn.

Fixed with commit d2c44289b50ca6fd5a031d94696e3b4910d3f771
The other drivers were drawing the same squares but much dimmer and harder to see.
The test was passing with softpipe by luck. If the test also probed the final color drawn to the window, it would have failed.